The term “cytokines” encompasses a diverse group of soluble proteins that are released by one type of cell and mediate a biological effect on another cell type. Biological activities exhibited by cytokines include control of proliferation, growth, and differentiation of various cell types, among which are cells of the hematopoietic or immune systems.
Examples of cytokines include the interleukins (e.g., interleulins 1 through 12), the interferons (IFNα, IFNβ, and IFNγ), tumor necrosis factor (TNFα and TNFβ), epidermal growth factor (EGF), platelet-derived growth factor (PDGF), and colony stimulating factors. Examples of colony stimulating factors (CSF), which control growth and differentiation of hematopoietic cells, are granulocyte-CSF (G-CSF), granulocyte-macrophage-CSF (GM-CSF), macrophage-CSF (M-CSF or CSF-1), mast cell growth factor (MGF), and erythropoietin (EPO).
The biological activity of cytokines generally is mediated by binding of the cytokine to a receptor specific for that cytokine, located on the surface of target cells. Much research has been directed to identifying receptor(s) that bind a given cytokine (often referred to as the “ligand” for the receptor in question), and exploring the roles that endogenous ligands and receptors play in vivo.
One family of cytokine receptors includes two different TNF receptors (Type I and Type II) (Smith et al., Science 248:1019, 1990) and Schall et al., Cell 61:361, 1990); nerve growth factor receptor (Johnson et al., Cell 47:545, 1986); B cell antigen CD40 (Stamenkovic et al., EMBO J. 8:1403, 1989); T cell antigen OX40 (Mallett et al., EMBO J. 9:1063, 1990); human Fas antigen (Itoh et al., Cell 66:233, 1991); and murine receptor 4-1BB (Kwon et al., Cell. Immunol. 121:414, 1989) [Kwon et al. I] and Kwon et al., Proc. Natl. Acad. Sci. USA 86:1963, 1989 [Kwon et al. II]).
Expression of murine 4-1BB is induced by concanavalin A (con A) in spleen cells, cloned helper T cells, cytolytic T cells, and cytolytic T cell hybridomas (Kwon et al. II). Murine 4-1BB cDNA was isolated from a cDNA library made from induced RNA isolated from both a helper T cell line and a cytotoxic T cell line (Kwon et al. II). The nucleotide sequence of the isolated cDNA is presented in Kwon et al. II, along with the amino acid sequence encoded thereby. The murine 4-1BB protein comprises 256 amino acids, including a putative leader sequence, trans-membrane domain and a number of other features common to cell membrane bound receptor proteins. Regarding a putative human 4-1BB protein, neither amino acid nor nucleotide sequence information is known for any such protein.
No ligand that would bind 4-1BB and transduce a signal through the 4-1BB receptor is known. Thus, there is a need in the art to determine whether a novel protein functioning as a ligand for 4-1BB exists, and, if so, to isolate and characterize the 4-1BB ligand protein.